1. Field of the Invention
The present invention relates to the field of telecommunications. More particularly, the present invention relates to reducing interference in code division multiple access (CDMA) communication systems.
2. Acronyms
The written description provided herein contains acronyms which refer to various telecommunications services, components and techniques, as well as features relating to the present invention. Although some of these acronyms are known, use of these acronyms is not strictly standardized in the art. For purposes of the written description, the acronyms are defined as follows:    Additive White Gaussian Noise (AWGN)    Binary Phase Shift Key (BPSK)    Code Division Multiple Access (CDMA)    Direct Sequence Code Division Multiple Access (DS-CDMA)    Inter-Symbol Interference (ISI)    Maximum Likelihood (ML)    Minimum Mean Square Error (MMSE)    Multi-Access Interference (MAI)    Multi-User Detection (MUD)    Single User Detection (SUD)    Orthogonal Variable Spreading Factor (OVSF)    Quaternary Phase Shift Key (QPSK)    Universal Mobile Telecommunications Systems (UMTS)    Wideband Code Division Multiple Access (W-CDMA)    Wide Sense Stationary Uncorrelated Scattering (WSSUS)
3. Background Information
The communications industry has experienced significant growth in the demand for wireless communications. The increased demand is related, in part, to the improved quality and reliability of wireless networks, including mobile cellular networks, which have essentially evolved through three generations. The first generation included analog systems that modulated voice signals onto radio frequency (RF) carrier waves, which were transmitted and received between base stations and mobile units. The second generation of cellular networks introduced digital encoding of analog voice signals, and included the time division multiple access (TDMA) and code division multiple access (CDMA) cellular systems. The second generation required a symmetric, full-duplex network and was directed to accommodation of voice traffic. The third generation of cellular systems includes packet-switched transmissions and can accommodate voice, data, audio and video communications.
With the increase in interest in commercial CDMA based mobile systems, as well as CDMA's dominance in third generation systems, research and development efforts have been directed to exploring receiver structures and detection techniques. CDMA systems belong to the genre commonly known as spread spectrum systems, the use of which is wide spread throughout wireless telecommunications. In spread spectrum systems, data from the transmitted signal is spread over a predetermined bandwidth, such that the transmission bandwidth of each transmitted signal is much higher than the actual data bandwidth. Therefore, numerous CDMA signals, originating from different users, are essentially spread and multiplexed onto the same transmission bandwidth. The signals corresponding to the different users are distinguishable by the CDMA receiver (or detector) based on the respective spreading codes or signatures.
CDMA systems include direct sequence CDMA (DS-CDMA), or direct sequence spread spectrum (DSSS), and frequency hopping CDMA. DS-CDMA, in particular, codes transmitted signals in sequential channels, combining the transmitted bits with a higher sampling rate chip sequence and spreading the signal according to a spreading ratio. Because each bit is, in essence, redundantly represented by multiple chips, a spread DS-CDMA data signal is more likely to be recovered by the receiver, even when portions of the data are lost or encounter interference during transmission. DS-CDMA includes various conventional coding schemes, such as IS-95, cdma2000 and wideband code division multiple access (W-CDMA).
The capacity of operational DS-CDMA systems is limited by cross-interference among the different user signals, all of which occupy the same frequency at the same time. The cross-interference among signals from different users is referred to as multi-access interference (MAI). Attempts to improve the capacity of CDMA systems included reducing the level of perceived interference among the users, including minimum mean square error (MMSE) detection. However, implementation of an adaptive MMSE detection scheme in universal mobile telecommunications systems (UMTS) requires major changes in the physical layer and the data link layer (layers 1 and 2, respectively) in the WCDMA air interface. Although a blind MMSE detection schemes (i.e., without training sequences) require only minor changes to W-CDMA air interface, they do not promise the kind of performance improvement as adaptive MMSE.
The receivers implemented in the second generation of CDMA systems were relatively simple matched filter based (correlation detectors) single user receivers. By the early 1990s, it became apparent that CDMA systems are not multi-access interference (MAI) limited. Rather, MAI was a limitation of the single-user Rake architecture incorporated into CDMA receivers. Therefore, the combined CDMA and Rake systems needed to be changed, or replaced, to minimize the MAI, as well as to account for internal signal interference (ISI), caused by other data bits transmitted from the same user.
Accordingly, the concept of multi user detection (MUD) was developed, by which a receiver jointly estimates all received signals from multiple transmission sources. However, an optimal MUD receiver, which is based on joint maximum likelihood (ML) estimation of all users, is impractical to implement because of the large computational complexity. MUD receivers are complex in CDMA systems because they must appropriately estimate the channels of all the users, as compared to a traditional single user detector, having the Rake architecture, which need only estimate a channel of a single user.
In a traditional CDMA system, the estimated parameters of the channel are hk(t) and τk, the amplitude/phase variation and the delay associated with each path, respectively. In a maximal ratio combining process, the receiver compensates for the delay and the amplitude/phase variation to obtain the maximum possible signal to noise ratio. An inherent limitation of the Rake architecture is its inability to suppress the residual cross-correlation between signals with different signatures.
The present invention overcomes the problems associated with the prior art, as described below.